Liquid crystals (LCs) have a remarkable ability to order, which is useful in electro-optical devices, such as electronic displays. In displays, for example, a thin layer of LC material is placed between glass plates and the orientation of the LC molecules is controlled by the application of an electric field with high spatial resolution. The order imparted on the LC molecules nearest the surface gets transferred through as many as 20,000 LC molecules with the result that the LC molecules furthest from the glass substrate still have the desired orientation. Ferroelectric liquid crystals (FLCs) and LCs subject to the electroclinic effect are most desirable in electro-optical devices, such as displays, switches, shutters, write heads for holographic data storage systems, and the like.
Ferroelectric liquid crystals (FLCs), which typically operate in the smectic C (SmC) phase, are most easily aligned when their SmC phase is overlaid by the nematic (N) and smectic A (SmA) phases. Thus, as the LC cools from the isotropic (I) phase, it first achieves monodirectional order in the N phase, to which is added layered order as it transitions to the SmA phase, to which is added tilted order as it transitions to the SmC phase. Hence, while FLCs require the presence of a very wide SmC phase in which to operate, they also require an N and SmA phase. Both the SmA and N phases should have a clear phase of at least 2° C., where the term “clear phase” refers to having only the desired phase, and no other coexistent phases, present in the cell over that temperature range.
To realize alignment uniform enough for display use, FLCs need not only the overlying phases, but adequate time in certain portions of the phases for the cell to reach equilibrium. For instance, with a traditional polyimide alignment layer, a very slow cooling rate is typically used throughout the SmA phase and the first few degrees of the SmC phase. That slow cooling rate would be extremely difficult to impose in a finished product. Consequently, FLC displays are typically restricted from going over any temperature that would result in a ruined product. Hence, most products containing FLC displays have both a quoted storage and operating temperature range, with the storage range being the temperature range the device can be subjected to, and the narrower operating range being the temperature range over which the device is expected to adequately perform. For most commercial purposes, a SmC to SmA transition over 90° C. is desired, and the material should retain its SmC phase down to less than −30° C., so a SmC phase width of over 120° C. is required. Having a higher SmC-SmA transition gives a clear commercial advantage.
U.S. Pat. No. 8,597,541, which is hereby incorporated by reference in its entirety, discloses that using certain types of thiadiazole compounds in mixtures, in conjunction with certain polarization-inducing components, considerably increases the polarization of LC mixtures compared with mixtures comprising the same proportion of the polarization-inducing component but lacking the thiadiazoles. In general, increasing the polarization of a mixture also increases its viscosity; the former will increase the mixture's switching speed, while the latter decreases the switching speed. Surprisingly, the polarization enhancement provided by the thiadiazoles of the '541 patent did not come with a commensurate increase in viscosity. This meant that the thiadiazole-based mixtures of the '541 patent, particularly those with a thiadiazole content comprising 30-50 weight percent of the mixture, had a faster switching speed than previous categories of FLCs. The present disclosure expands upon the work described in the '541 patent.